Over the past thirty years, significant time and effort has been dedicated to the study of various materials such as, LiNbO3, III-V semiconductors, and organic polymers, to determine their respective characteristics for the fabrication and performance as electro-optic (EO) devices (e.g., modulators, waveguides, switches, emitters, detectors, and the like). For example, organic modulators have been demonstrated at operating frequencies as high as 113 GHz and a Vπ of approximately 0.8V. However, for applications to Radio-Frequency Photonics (RF Photonics) in which a high-frequency electrical signal is transmitted not via a metallic transmission line but via an optical fiber, a much lower modulation voltage of around 0.5V or smaller is required. It is well known that the power consumption of an electro-optic modulator is proportional to the square of the modulation voltage. Hence, other than RF Photonics applications, it is generally desirable to lower the switching or modulating voltage of an electro-optic modulators for the purpose of saving driving power. In addition, lowering the driving power also leads to lower heat dissipation, which will help in achieving higher-density device integration.
For a given electro-optically active material, the prior high-speed electro-optic device structures are limited in their ability to provide low voltage operation. Present high-speed electro-optic devices utilize metals as the voltage-conducting electrodes. Metals, however, are very absorptive with respect to an optical beam propagating within the electro-optically active material. This limits how close the metal electrodes can be, which subsequently limits the strength of the electric field achievable at the electro-optically active material and, hence, the modulator voltage. The present electro-optic modulator structures are thus incapable of achieving much lower modulation voltages without incurring higher optical losses. Such considerations make it difficult to achieve low modulation voltage, high integration density, or smaller device sizes, and consequently limit applications of such devices